1. Field of the Invention
This invention relates to an apparatus and method for surface treatment which includes a fluidized bed furnace. More particularly, it is an apparatus and method which can prevent the agglomeration of a powder forming a fluidized bed which will obstruct its uniform fluidization, and can also protect a gas distributor from a highly reactive powder, or atmosphere.
2. Description of the Related Art
There are known methods which employ a fluidized bed to form a carbide, nitride, or other surface layer on the material to be treated, as described in U.S. Pat. No. 4,844,949 which cover prior inventions of the inventors of this invention.
According to these methods, a fluidized bed furnace is supplied with a fluidizable powder mixture (hereinafter referred to as fluidized bed powders) consisting of a powder of alumina, or other refractory material (hereinafter referred to as a fluidizing agent), and a powder of a metal, or alloy, or both for forming a carbide, nitride, or other surface layer (hereinafter referred to as a surface layer-forming powder), and a fluidizing gas, such as argon, is introduced into the furnace to fluidize the fluidized bed powders to form a fluidized bed. The material to be treated is placed in the fluidized bed, and the bed is heated, while a halide is appropriately supplied into the bed, whereby a surface layer is formed on the material to be treated.
The reaction of the halide and the surface layer-forming powder produces the gas of a halide of the surface layer-forming element, and the reaction of the halide gas with carbon or nitrogen in the material to be treated forms the surface layer. It has, however, often been the case that during the process of formation of the surface layer, the agglomeration of a part of the fluidized bed powders occurs to form a mass in the fluidized bed, depending on the conditions employed for surface treatment, including the amount of the halide gas produced and the flow rate of the fluidizing gas. If any such mass is formed in the fluidized bed, the powders around the mass fail to be satisfactorily fluidized, and the fluidized bed lacks uniformity in temperature, and has, therefore, only a limited area in which it can form a uniform surface layer on the material to be treated. Moreover, the mass often interferes with a jig employed for holding the material to be treated in the furnace, and thereby disables it to be held in a proper position. Therefore, it is often necessary to interrupt the operation for surface treatment to remove any such mass.
The agglomeration of the powders in the fluidized bed can be diminished by increasing the flow rate of the fluidizing gas to achieve a higher degree of fluidization, or by decreasing the amount of the halide to be added. Both of these steps are, however, impractical, since the increased consumption of the fluidizing gas adds to the cost of the surface treatment, and the changes in the necessary conditions for the treatment, including the concentration of the reactive gas in the fluidized bed, result in the formation of a surface layer having an undesirably small, or uneven thickness.
The furnace employed by the known methods is provided with a gas distributor for distributing the fluidizing gas through the fluidized bed. The gas distributor is, however, in contact with the fluidized bed, and is, therefore, exposed to the halide gas in the fluidized bed. It is very likely that the gas distributor may be corroded by the halide gas, or that the fine openings thereof may be clogged with the reaction product. Such corrosion and clogging lower the function of the gas distributor.